The invention relates to a component operating with bulk acoustic waves, and particularly to a stacked crystal filter with thin-film resonators that are stacked one on top of the other and are connected to one another (or Thin Film Bulk Acoustic Wave Resonator, “FBAR”), which is also referred to as a Bulk Acoustic Wave resonator (or BAW resonator).
Resonators such as these are particularly suitable for bandpass radio-frequency filters in modern filter technology and may, for example, be used in mobile communication appliances.
A resonator operating with bulk acoustic waves has at least one piezoelectric layer that is arranged between two metal layers (electrodes). In a stacked crystal filter, two or more resonators are stacked one on top of the other, for example by a second piezoelectric layer being deposited first of all on the uppermost electrode of the lower resonator, after which an upper electrode is deposited for the upper resonator. In this case, the upper and the lower resonator have a common electrode, and are acoustically coupled to one another.
It is known that a BAW resonator can be provided with an acoustic mirror, which is preferably arranged between a mechanical mount substrate and the BAW resonator. The acoustic mirror comprises alternating layers which respectively have a low and a high acoustic impedance, with their layer thicknesses in each case being approximately one quarter of the wavelength of the acoustic primary mode (related to the speed of propagation of the acoustic wave in the respective material). The acoustic mirror thus represents one or more boundary surfaces, which, at the resonant frequency, reflect the acoustic wave back into the resonator, and prevent the wave from emerging in the direction of the mount substrate.
A partially permeable acoustic mirror may be arranged between the upper and the lower resonator of the stacked crystal filter. The partially permeable acoustic mirror forms a coupling layer system, and can be used to control the acoustic coupling between the two resonators.
When radio-frequency filters are being produced for mobile communication applications, it is often desirable in order to reduce the space requirement for the filter at the same time to act as the Balun, with an asymmetric signal being processed to perform antiphase signals at a symmetrical output.
FIG. 1 shows, schematically, a known stacked crystal filter with an integrated Balun functionality. A first resonator RE1 is formed by a first piezoelectric layer PS1, a first electrode E1 and a second electrode E2, and is connected between a signal-carrying connection T1 of a first, asymmetric electric port and ground.
A second resonator RE2 is formed by a second piezoelectric layer PS2, a third electrode E3 and a fourth electrode E4, and is connected between a first connection T21 and a second connection T22 of a second, symmetrical electrical port.
The resonators RE1 and RE2 are acoustically connected to one another by way of a coupling layer system KS. The coupling layer system KS has alternately arranged mirror layers (which are partially permeable in the acoustic sense) with a high and a low acoustic impedance, respectively, (reference symbols HZ and LZ, respectively), which preferably have a thickness of λ/4, where λ is the wavelength of the acoustic wave to be reflected.
This stacked crystal filter has the disadvantage that the components of the acoustic wave which are reflected on different boundary surfaces of the mirror layers of the coupling layer system have a phase difference between them and thus, in particular, contribute to the discrepancy in the phase difference of the symmetrical signals at the second port of about 15 to 20° from the desired phase difference value of 180°. Furthermore, this leads to varying amplitudes of the signals at the symmetrical port, which has been found to be disadvantageous for the functionality of the stages of an electronic circuit (for example, of an amplifier) to be connected downstream.